Zinc oxide varistors are ceramic semiconductor devices based on zinc oxide. They have highly non-linear current/voltage characteristics, similar to back-to-back Zener diodes, but with much greater current and energy handling capabilities. Varistors are produced by a ceramic sintering process which gives rise to a structure consisting of conductive zinc oxide grains surrounded by electrically insulating barriers. These barriers are attributed to trap states at grain boundaries induced by additive elements such as bismuth, cobalt, praseodymium, manganese and so forth.
The electrical characteristics of a metal oxide varistor, fabricated from zinc oxide, are related to the bulk of the device. Each zinc oxide grain of the ceramic acts as if it has a semiconductor junction at the grain boundary. The non-linear electrical behavior occurs at the boundary of each semiconducting zinc oxide grain. Accordingly, the varistor can be considered as a multi-junction device, composed of many series and parallel connections of grain boundaries. The device behavior may be analyzed with respect to the details of the ceramic microstructure. Mean grain size and grain size distribution play a major role in electrical behavior.
Fabrication of zinc oxide varistors has traditionally followed standard ceramic techniques. The zinc oxide and other constituents are mixed, for example by milling in a ball mill, and are then spray dried. The mixed powder is then pressed to the desired shape, typically tablets or pellets. The resulting tablets or pellets are sintered at high temperature, typically 1,000.degree. to 1,400.degree. C. The sintered devices are then provided with electrodes, typically using a fired silver contact. The behavior of the device is not affected by the configuration of the electrodes or their basic composition. Leads are then attached by solder and the finished device may be encapsulated in a polymeric material to meet specified mounting and performance requirements.
In the device thus fabricated, the bulk of the varistor between its contact or electrode layers thus consists primarily of zinc oxide grains of a predetermined average grain size, yielding a specific resistivity per unit of thickness dimension. In designing a varistor for a given nominal varistor voltage, it is therefore basically a matter of selecting a device thickness such that the appropriate number of grains is in series between the electrodes. The voltage gradient of the varistor material, in terms of volts per unit of thickness dimension, can be controlled by varying the composition and manufacturing conditions of the varistor. Altering the composition of the metal oxide additives enables the grain size to be changed for this purpose. In practice, the voltage drop per grain boundary junction is approximately constant and does not vary greatly for grains of different sizes. Accordingly, varistor voltage is primarily determined by the thickness of the material and the size of the grains.
The construction and performance of varistors is discussed inter alia in "Zinc Oxide Varistors--A Review" by L. M. Levinson and H. R. Philipp, Ceramic Bulletin, Volume 65, No. 4 (1966), which article may be referred to for further detail.
A multiplicity of specific varistor compositions are known and described, inter alia, in the following patent specifications: U.S. Pat. Nos. 3,598,763; 3,663,458; 3,863,193; 4,045,374 and GB 1,478,772. Methods of manufacturing varistors are described, inter alia, in U.S. Pat. No. 3,863,193 and 4,148,135.